Intravascular optical coherence tomography (OCT) systems operate via a rotating catheter that is introduced into a patient's blood vessels to image the vessel walls to detect atherosclerotic plaque and other life-threatening conditions. The catheter takes a picture by shooting beams of light as it rotates and translates within the vessel and detecting light that is reflected back. The light is detected in the form of a series of lines arranged in a helix. A computer processor can digitize the information and arrange it for presentation on a screen.
One problem in OCT is calibration. The lines are treated as if they originate at the center of the fiber optical catheter, but this is based on assumptions about the path length of light through the catheter. The catheter is, in-fact, subject to stresses that can distort the path length during imaging. Thus, if the catheter stretches by a millimeter during imaging, the radius to a point on each line is artificially shrunken by a millimeter. If a diameter is measured on that image, the diameter will be wrong by two millimeters.
Unfortunately, the fiber optical catheter is within a patient during imaging and the imaging proceeds rapidly, so it is not realistic for an attendant to stand by and measure stretching or distortion of the catheter. Since one measure of the severity of atherosclerosis is cross-sectional area of space within a blood vessel, path length changes in OCT imaging hardware interfere with accurately evaluating the severity of a patient's condition.